An airborne moving target indicator (AMTI) radar is generally known and is the type of radar that has the capability to reject or cancel signals from fixed, or unwanted targets (non-movers), such as buildings, hills, etc. At the same time, such radars typically highlight or enhance the radar return signals from any moving targets (movers) such as aircraft, vehicles, or the like. One technique used in AMTI radar of the coherent type involves utilizing the doppler shift imparted to the reflected radar signals by a moving target as a part of a processing scheme to distinguish a mover from a non-mover, This doppler shift appears as a change in the phase of the received signals between consecutive illuminating radar pulses.
There are a number of problems which must be considered in the processing of radar returns where the AMTI radar is mounted in an aircraft. Because the aircraft is moving with respect to both the fixed and moving targets, the radar returns from both target and clutter experience a frequency shift which can be corrected by known motion compensation techniques.
Synthetic-aperture radars are also generally known and such system generally use a multi-aperture antenna and movement of the platform on which the antenna is mounted as additional input into the processing of return signals in an AMTI radar. While this adds significantly to the complexity of the processing of the radar return signals, the resultant clutter cancellation can significantly enhance the identification of moving targets.
One well-known method of compensating for the effects of aircraft motion is known as displaced phase center technique and involves electronically displacing the antenna's phase center along the flight path of the aircraft. Briefly, the technique involves the transmission and reception of radar returns by the antenna of the radar system having its phase center at a first known location. A second illuminating pulse is then transmitted and the return stored while the antenna has its phase center at a second known location. The phase centers of the first and second returns are separated by a precisely known distance related to the movement of the aircraft during the interpulse period and, knowing this information, the phase centers can electrically be changed to essentially coincide in time. At that point, the signals received by the multiaperture antenna from clutter, or stationary objects, will have properties suitable to cancellation leaving only the movers to be detected.
One technique for clutter cancellation is described in U.S. Pat. No. 4,093,950 issued Jun. 6, 1978 to T. ap Rhys for MOTION-COMPENSATION ARRANGEMENTS FOR MTI RADARS. The clutter suppression technique described in this patent is not limited to two pulses at a time but may be applied to a number of pulses. Phase and amplitude adjustments are also made to minimize the effects of antenna construction errors. The antenna subarrays have phase centers which are separated by 2VT. The sum and difference signals from each to adjacent subarray are taken to produce a sum channel and a difference channel for each group of subarrays. After adjustment of the difference channel signal in phase and amplitude, the latest return is added to a delayed return to produce a correction signal. That correction signal is then added to a delayed signal in the corresponding sum channel to provide a signal that is synchronized in time and phase with the most recent signal in the sum channel.
Also of interest is an article entitled "Air-to-Ground MTI Radar Using a Displaced Phase Center, Phased Array" by M. L. Stone and W. J. Ince published June 1980 in the IEEE International Radar Conference Proceedings. The article describes an AMTI radar utilizing an electronically scanned displaced phase center antenna, an advanced digital signal processor and real time, automated target acquisition and display. In this approach, the first pulse of a pair of transmissions is radiated from the leading phase center and the received radar signal is stored in a buffer. The second pulse is radiated a short time later from the rear phase center when the boresite-axis is precisely aligned with the boresite direction of the first transmission. The second received signal is also subtracted from the first received signal in an MTI canceller circuit. The pulse pair separation is equal to D/V, where D is the phase center displacement and V is the aircraft velocity, the clutter is eliminated by the canceller circuit, enabling slow moving targets to be detected. It should be noted that the antenna phase center is translated along the direction of the array axis by a single wavelength to compensate for the forward motion of the aircraft. This is accomplished by a radiating element switching circuit which alternates on a pulse-by-pulse basis.